The uninterruptible power supply (or UPS) is an emergent power supply device connected between a power source and a load, in which the power source can be a commercial AC power supply or any other AC power source. The purpose of an UPS is to ensure the normal operation for a load device when the power source is malfunctioned or becomes abnormal.
FIG. 1(a) shows a circuit block diagram of a conventional uninterruptible power supply. The uninterruptible power supply as shown in FIG. 1(a) includes a battery charger 10, a battery module 11, a DC-DC converter 12, an inverter 13, and a switch circuit 16. The uninterruptible power supply of FIG. 1 is configured to deliver the electric power received from a commercial power supply 15 to a load 14, or deliver the electric power obtained from the conversion of the electric power outputted from the battery module 11 to the load 14 when the commercial power supply 15 becomes abnormal. The battery charger 10 includes a transformer 101, a rectifier 102, a filter bank 103, a voltage regulator 104, a switch 105, a CPU and a current-limiting device 106.
When the commercial power supply 15 is operating normally, the CPU enables the switch circuit 16 to be connected to the commercial power supply 15 so that the AC voltage Vin is delivered to the load 14 for powering the load 14. Also, the AC voltage Vin is delivered to the transformer 101 of the battery charger 10 to undergo a down-conversion process so as to be converted into an AC voltage with a lower voltage level. The converted AC voltage is rectified into a DC voltage through the rectifier 102 and the resultant DC voltage is processed to filter its noise and ripple through the filter bank 103, so as to obtain a stable DC voltage V1.
The voltage regulator 104 is connected to the switch 105 which is manipulated by the CPU for regulating the input voltage of the battery module 11. The current-limiting device 106 is connected to an output terminal of the voltage regulator 104 and an input terminal of the battery module 11 for limiting a maximum value of a charging current for the battery module 11.
When the commercial power supply 15 is abnormal, the CPU manipulates the switch circuit 16 to turn off and enables the battery module 11 to supply power. The DC voltage Vout outputted from the battery module 11 is boosted through the DC-DC converter 12, and the boosted DC voltage is delivered to the inverter 13. The inverter 13 is configured to convert the boosted DC voltage into an AC voltage and deliver the AC voltage the load 14.
Generally speaking, the AC voltage outputted from the inverter 13 within the conventional uninterruptible power supply has a square waveform or a sinusoidal waveform under the Battery-Backup mode. As shown in FIG. 1(b), a circuit block diagram of the inverter 13 of FIG. 1 is illustrated. In FIG. 1(b), one end of inverter 13 is coupled to the DC-DC converter 12 and the other end of the inverter 13 is coupled to the load 14. The load 14 is made up of a load resistor 141 and a load capacitor 142. The load 14 generally includes capacitive elements because the load 14 is a rectified RC load.
The inverter 13 includes four switches S1, S2, S3, S4. FIG. 1(c) is a timing diagram showing the switching sequence of the switches S1, S2, S3, S4. The voltage waveform of the AC voltage Vout generated across the load resistor 141 and the load capacitor 142 is generated by the switching mechanism of the switches S1, S2, S3, S4.
The switching mechanism of the switches S1, S2, S3, S4 is conducted in an alternate way between the positive half-cycles and the negative half-cycles. As an example, the switches S1 and S4 are turned on and the switches S2 and S3 are turned off during the positive half-cycle of the period of t0 to t1, and the switches S1 and S4 are turned off and the switches S2 and S3 are turned on during the negative half-cycle of the period of t2 to t3, and thereby generating an output AC voltage Vout across the load resistor 141 and the load capacitor 142. Because the inverter 13 employs an alternate switching mechanism to generate the AC voltage Vout between the positive half-cycles and the negative half-cycles, the voltage across the load resistor 141 and the load capacitor 142 must be zeroed at the point of transition from the positive half-cycle to the negative half-cycle and the point of transition from the negative half-cycle to the positive half-cycle, so that the switching loss of the inverter 13 can be minimized. However, the switching mechanism of the switches S1, S2, S3, S4 within the inverter 13 will force the load capacitor 142 to be charged and discharged during each half-cycle of the AC voltage. Therefore, at the point of transition from the positive half-cycle to the negative half-cycle or from the negative half-cycle to the positive half-cycle, the load capacitor 142 will be fully discharge to zero to prevent the output voltage with DC level if the resistance of the load resistor 141 is too large. A known solution to address these deficiencies is attained by allowing the switches S1 and S3 to turn on or allowing the switches S2 and S4 to turn on during the time interval between a positive half-cycle and a negative half-cycle, for example, the time interval of t1 to t2 or the time interval of t3 to t4. The switching mechanism of FIG. 1(c) allows the switches S2 and S4 to turn on to form a discharge loop among the load capacitor 142 and the switches S2 and S4, thereby the stored energy of the load capacitor 142 will be dissipated by the switches S2 and S4.
However, the aforementioned solution is prone to consume a great amount of power and arise the temperate of the switches S1, S2, S3, S4. Consequently, a large heat-dissipating device is required to be attached to the surface of the switches. Also, the dissipated energy normally implies the deterioration of the efficiency of the inverter, and the discharge time of the battery module will be shorter accordingly.
FIG. 2 shows another representation of a conventional uninterruptible power supply. As shown in FIG. 2, the uninterruptible power supply is made up of a first switch circuit 21, a transformer 22, a rectifier 23, a filter bank 24, a voltage regulator 25, a switch 26, a CPU, a current-limiting device 27, a battery module 28, an inverter 29, a second switch circuit 30 and a RC circuit 31. The uninterruptible power supply of FIG. 2 is configured to supply the power from the commercial power supply 32 to a load 33 or the power obtained from the conversion of the electric power stored in the battery module 28 to the load 33 when the commercial power supply 32 becomes abnormal. The transformer 22 is made up of a first winding N1, a second winding N2, a third winding N3 and a fourth winding N4, and the load 33 is made up of a load resistor 331 and a load capacitor 332.
When the commercial power supply 32 is operating normally, the CPU enables the first switch circuit 21 to be connected to the commercial power supply 32 so that the AC voltage Vin is connected to the load 33 and the winding N1. In the meantime, the first winding N1 serves as a primary winding, while the second winding N2, the third winding N3 and the fourth winding N4 collectively serve as a secondary winding assembly. The AC voltage Vin received by the first winding N1 is transferred to the secondary winding assembly through electromagnetic action so that the second winding N2 can output an output voltage V2 and the third winding N3 can output an output voltage V3.
The second winding N2 is provided with the capability of automatic voltage adjustment. The voltage connected to the load 33 is adjusted based on the connection status of the second switch circuit 30 so as to meet the specified operative voltage for the load 33. Therefore, the bad power supply voltage on the operation of the load 51 due to the over/under voltage condition occurred to the AC voltage delivered to the load 33 can be avoided.
The voltage V3 outputted from the third winding N3 is provided to charge the battery module 28 through a battery charger consisted of a rectifier 23, a choke L, a filter bank 24, a voltage regulator 25, a switch 26, and a current-limiting device 27, in which the choke coil is an optional device. The design norm and the operation theorem of the aforementioned circuit components within the battery charger have been described exhaustively in the above statements with reference to FIG. 1, and it is not intended to give details herein.
If the commercial power supply can not operate normally, the CPU manipulates the switch circuit 21 to turn off and enables the battery module 28 to supply power. The DC voltage from the battery module 28 is delivered to the inverter 29 and converted into an AC voltage to the fourth winding N4. The AC voltage received by the fourth winding N4 is transferred to the first winding N1 and the second winding N2 through electromagnetic action and boosted by the first winding N1 and the second winding N2. The voltage induced across the first winding N1 & the second winding N2 serve as the operative power for the load 33, and adjusted according to the connection status between the second winding N2 and the second switch circuit 30, thereby avoiding the bad output voltage on the operation of the load 33 due to the over/under voltage condition occurred to the voltage delivered to the load 33.
Although the circuitry of FIG. 2 is sufficient to solve the foregoing problems incurred with the circuitry of FIG. 1, some imperfections are still unresolved and need to be removed. For example, the transformer 22 that is taken as a voltage boosting elements for the uninterruptible power supply of FIG. 2 would produce a leakage inductance among the coil windings. Therefore, the transformer would release the energy stored in the leakage inductance when the internal switch (not shown) of the inverter 29 (not shown) transits from the turn-on state to the turn-off state. This would cause instantaneous spikes to the output voltage of the first winding N1. A known solution to remove the spikes occurred to the output voltage of the first winding N1 is to place a RC circuit 31 across the first winding N1 for suppressing the spikes occurred to the output voltage. Unfortunately, the RC circuit does little improvements to the suppression of the voltage spikes.
In addition, when the commercial power supply can not operate normally, the fourth winding N4 of the transformer 22 that serves as a voltage boosting device have to suffer large current variation. Therefore, the fourth winding N4 have to be manufactured by copper wires with a larger thickness. In this way, the overall size of the transformer would become very large and the copper wires would dissipate a great amount of energy, thereby deteriorating the efficiency of the uninterruptible power supply.
There is a need to develop an uninterruptible power supply that can tackle the foregoing problems and imperfections encountered by the prior art.